Modulators are used for signal transmission in wireless or wireline communication systems. One of the functions of a modulator is to modulate information onto a carrier frequency signal in order to provide a transmission signal. The transmission signal is amplified before being provided to a transmission channel.
In typical transmitters, digital baseband information that is to be transmitted is first converted by a digital-to-analog converter (DAC) into analog information. The DAC may be an R-string or a current steering DAC. In order to attenuate the out-of-band quantization noise of the DAC, the output of the DAC is provided to a resistor-capacitor (RC) filter. The output of the RC filter is converted into a current by a voltage/current converter. The current is applied to the source of a multiplier-based differential up-conversion mixer pair. The gates of the mixer pair are driven by a frequency signal provided by a local oscillator (LO). The frequency signal is chosen to be at the desired radio frequency of the transmitter. This approach requires that the voltage/current converter have high linearity which tends to increase the power consumption of the transmitter. That is, as the linearity requirement for the voltage/current converter increases, the quiescent current of the transistors within the converter with respect to the modulated current increases. High linearity for the voltage/current converter can be difficult to achieve if the transistors have non-linear characteristics.
For these and other reasons there is a need for the present invention.